This invention relates to spread-spectrum communications and more particularly to a broadband code division multiple access communications system which communicates over the same frequency band of an existing frequency division multiple access (FDMA), proposed time division multiple access (TDMA) or any other mobile-cellular system.
The current mobile-cellular system uses the frequency band 868-894 MHz for transmission from the mobile unit to the cellular base stations and the frequency band 823-849 MHz for transmission from the cellular base stations to the mobile unit. Each of these frequency bands is divided in half to permit two competitive systems to operate simultaneously. Thus, each system has 10.0 MHz available for transmission and 10.0 MHz for reception. Each of the 10.0 MHz bands is divided into 30 kHz channels for voice communications.
A problem in the prior art is limited capacity due to the number of channels available in the mobile radio cellular system.
FIG. 1 is a diagram of the cellular system. A mobile unit serviced by cell A located near the border of cells A and B and a mobile unit serviced by cell B located near the same border are received by the cellular base stations of cells A and B with almost the same power. To avoid interference between units operating in the same frequency band at comparable power levels, different frequency subbands, i.e. channels, are allocated to adjacent cells. FIG. 1 shows a seven frequency scheme, with each cell having a bandwidth equalling 10.0 MHz/7, which approximately equals 1.4 MHz. This frequency scheme has adjacent cells operating at different frequencies, thereby reducing interference among units in adjacent cells. This technique is called frequency reuse. As a result of frequency reuse, each cell has N=1.4 MHz/30 kHz=46 channels, with the channels divided and allocated as shown in FIG. 2. The 1.4 MHz is divided for FDMA or TDMA into 30 kHz channels for communications, each with a 180 kHz guard band. Each of the different cells, A through G, have channels that lie in a different 30 kHz band so that their spectra do not overlap. FIG. 2 also shows the spread-spectrum signal overlaying on the existing units whether they be Advanced Mobile Phone Service (AMPS) or IS54. Some of these channels are reserved for signalling, leaving approximately 41 channels per cell. The channels are allocated to cells A, B, and C as shown in FIG. 2. A guard band of 180 kHz separates each channel so that adjacent channel units within the same cell do not interfere with one another.
The cells in a mobile-cellular system are expensive to maintain, and profitability can be significantly increased by increasing the number of units per cell. One approach to increase the number of units per cell is to change from analog frequency modulation (FM) communication, and to use digital communication with time division multiple access.
Existing AMPS cellular systems exhibit numerous deficiencies; for example, lack of privacy, dropped calls, low data rate capabilities, poor quality speech, and limited capacity. TDMA and code division multiple access (CDMA) proposals attempt to overcome the capacity and privacy issues, with TDMA systems proposing to pack either 3 or 6 units in each 30 kHz frequency band previously used by the AMPS FM unit, while narrowband CDMA proposals, for example, by Qualcomm, claim a further increased capacity.
An object of the invention is a spread-spectrum communications network for increasing capacity for communications in a mobile radio cellular system environment.
Another object of the invention is a spread-spectrum system which can be used at the same frequencies as used for the mobile radio cellular systems.
An additional object of the invention is a spread-spectrum system which can be used concurrently with a mobile-cellular system without interfering with the mobile-cellular system.
A further object of the invention is a spread-spectrum system which allows communications between base units and spread-spectrum units.
A still further object of the invention is a spread-spectrum system which can overlay geographically and overlay in spectrum, on an already existing mobile-cellular system, without modifications to the mobile-cellular system, including FDMA-AMPS systems and TDMA-IS54 cellular systems.
According to the present invention, as embodied and broadly described herein, a broadband code division multiple access (B-CDMA) spread-spectrum communications system for communicating data between a plurality of spread-spectrum units is provided, comprising a plurality of spread-spectrum-base stations and a plurality of spread-spectrum units. The spread-spectrum units communicate through the spread-spectrum-base station. Data may be, but are not limited to, computer data, facsimile data or digitized voice.
The spread-spectrum CDMA communications system is located within a same geographical region, i.e. cell, as occupied by a mobile-cellular system. Typically, the cellular-base station and the spread-spectrum-base station are collocated. Each cell of the mobile-cellular system has a cellular bandwidth. Typically, the cellular bandwidth is 10.0 MHz. The cellular bandwidth is divided into a plurality of predetermined channels. The predetermined channels are separated by guard bands. The mobile-cellular system has mobile-cellular units communicating on the predetermined channels.
A plurality of spread-spectrum-base stations communicate over the same geographical region as occupied by the mobile-cellular system. A spread-spectrum-base station communicates data between the plurality of spread-spectrum units.
Each spread-spectrum-base station has base-converting means, base-spread-spectrum-processing means, base-transmitting means, a base antenna, and base-detection means. Each spread-spectrum-base station additionally has base-comb-filter means and/or base-sector means. The base-converting means converts the format of the data to be transmitted to a spread-spectrum unit into a form for communicating over radio waves. The base-spread-spectrum-processing means processes the converted data with spread-spectrum modulation. The base-transmitting means transmits across the cellular bandwidth, from the spread-spectrum-base station to a spread-spectrum unit, the spread-spectrum-processed-converted data. The base-comb-filter means filters, or attenuates, i.e. notches-out, the predetermined channels of the mobile-cellular system. The base-sector means sectors, using directional antennas, the geographic coverage of the spread-spectrum-base station into two or more sectors. The base-detection means is coupled through the base-comb-filter means and/or the base-sector means to the base antenna. The base-detection means recovers data communicated from the spread-spectrum unit to the spread-spectrum-base station.
The plurality of spread-spectrum units are located in the cell. Each of the spread-spectrum units has a unit antenna and unit-spread-spectrum-detection means. The unit-spread-spectrum-detection means recovers data communicated from the spread-spectrum-base station. For communicating to the spread-spectrum-base station, the spread-spectrum unit has unit-spread-spectrum-converting means, unit-spread-spectrum-processing means and unit-spread-spectrum-transmitting means. The unit-spread-spectrum-converting means converts the format of data from a spread-spectrum unit into a form for communicating over radio waves. The unit-spread-spectrum-processing means processes the data with spread-spectrum modulation. The unit-spread-spectrum-transmitting means transmits across the cellular bandwidth, the spread-spectrum-processed converted data from the spread-spectrum unit to a spread-spectrum-base station.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.